A rational synthetic method is developed to produce monodisperse metal sulfide nanocrystals (NCs) in organic non-polar solutions by using (NH(4))(2)S as a sulfide precursor. (NH(4))(2)S is stabilized in an organic primary amine solution and exhibits high reactivity toward metal complexes. This novel technique exhibits wide applicability for the organic phase metal sulfide NC synthesis: a large variety of monodisperse NCs have been synthesized, including Cu(2)S, CdS, SnS, ZnS, MnS, Ag(2)S, and Bi(2)S(3). The stoichiometric reactions between (NH(4))(2)S and metal salts afford high conversion yields, and large-scale production of monodisperse NCs (more than 30 g) can be synthesized in a single reaction. The high reactivity of (NH(4))(2)S enables low temperature (< 100 (o)C) syntheses, and the air-stable materials (such as CdS NCs) can be produced in air. Moreover, this low-temperature technique can be used to produce small size NCs which are difficult to be synthesized by the conventional high temperature methods, such as sub-5nm Ag(2)S and Bi(2)S(3) quantum dots.